Director Miguel Sapochnik was at the helm for season five's "Hardhome" which, prior to "Battle of the Bastards," was one of the biggest man-to-man fight sequences the series has done. But Hardhome had about 100 fewer extras, and no horses in sight. Sapochnik returned in season six to direct both "Battle of the Bastards" and the upcoming season six finale.

Part of the magic behind the fight sequence came from all the physical work put into staging this war. Visual effects were used to duplicate the extras or show arrows flying through the air, but a vast majority of the action on screen was played out in real life.

Jon Snow and Tormund on set atop a body of bodies from Sunday's bloody "Game of Thrones" episode.HBO

"Probably my favorite shot of the whole season is when we're behind Jon Snow and he sees that cavalry wall galloping towards him," showrunner David Benioff said in a behind-the-scenes video. "And part of the reason it's such a great shot is it's all real. That's 40 horses charging full speed at Kit Harington."

"Until the last minute, I ... stood there facing off against this calvary charge which is really scary," Kit Harington (Jon Snow) said. "We were a bit annoyed because I think everyone's going to think it was CGI and it wasn't."

The "horse whisperer" on set, Camilla Naprous, had to choreograph the calvary charge carefully to help make it appear as though they were crashing into one another. The shot was a fine balance between making the action look real, and keeping the horses and stuntmen (and women) safe.

"We make channels so it looks like they're clashing but they're not, the horses are actually just passing through two foot channels so they can clear out," Naprous explained. She worked alongside Rowley Irlam, the "Game of Thrones" stunt coordinator.

"So in very tight formation we'll have those guys cross, and they will pull the horses as they cross through," Irlam explains. "We're falling them onto thick falling beds so we don't injure the horses or injure the guys."

Even the smaller moments from the episode, like Jon Snow beating the ever-living crumb out of Ramsay, had immense efforts behind them. "We actually spent an entire day, ten hours, with Kit on top of Iwan, beating him," Sapochnik explained. "And I just shot it from every single angle I possible could."

And all this is just the tip of the iceberg. For a full look behind the scenes of the "Battle of the Bastards," watch the mini-documentary released by HBO below: